Did the Buddha teach receivers to be ungrateful?
No, of course not. The quote that you refer to is using a hyperbolic statement, “It's the donor who should express the gratitude for the opportunity to build up his great kammic storehouse,” to make the point vividly that true generosity benefits everyone, both the giver and the receiver. The key point being made, though, is the first line of the quote: “If you truly understand the practice of generosity, then you'd never have to look for outright expression of gratitude from the receivers.” This is key, because if one acts generously in order to receive gratitude from the receiver, then one is not being authentically generous, but is merely flattering their ego, or attempting to garner social standing, or even, perhaps, seeking to elevate themself above the receiver who was in need. As the gift is done in expectation of a return, it is only a transaction, and as such, binds one to the mundane world of suffering. It neither benefits the giver, nor the receiver, in a spiritually significant way. The Buddha would never have promoted transactional generosity.